Study On The Structure Design And Lithium-storage Performance Of Transition Metal Compound Anodes

With the rapid development of society and economy,the demands for energy are keeping growing.However,energy shortages and environmental issues have led to an increasing urgency for the development of new energy resources.Currently,as a dominant energy storage system,lithium-ion batteries have shown the great advantages of high energy density,no memory effect,long cycle life and environment-friendly,which are widely applied in various fields,such as mobile devices and electric vehicles.In addition,with the increasing demand for foldable electronic units and wearable devices,portable energy storage systems are developing in the direction of light-weight,miniaturization and flexibility,so it’s of great significance to the development and design of flexible electrodes.The commercial anode now is graphite who possesses low specific capacity,large expansion effect and poor rate capability,which limits the improvement of high-performance lithium-ion batteries.As a result,it has beome a hot topic to find alternatives to graphite inrecent years.Among them,transition metal compounds take advantages of high theoretical capacity,low cost and simple process,which are potential to become the next-generation lithium-ion battery anodes.However,they produce large volume expansion during charge/discharge processes,which results in the structural damage and fast capacity decay.Besides,the poor electrical conductivity of transition metal compounds leads to poor rate capability.This text aims at the issues like volume expansion and poor conductivity of transition metal compounds,in order to control the reaction kinetics and ion transport path of the anode,some solutions are proposed such as nano-structure design and composite preparation to complement the benefits and drawbacks,contributing to the enhanced electrochemical performance.The lithium storage mechanism and kinetic characteristics of the anode materials are also discussed deeply.The details are described as follows:（1）Aiming at Mn₃O₄ material,Mn₃O₄ hollow polyhedrons with controlled morphology were designed and prepared by a one-step hydrothermal method.The unique morphology and strong framework effectively enhance the structure stability of the electrode.Electrochemical results show that the as-prepared Mn₃O₄@C anode exhibits an excellent electrochemical performance.The initially reversible capacity is 1254.1 m Ah g^-1 at a current of 100 m A g^-1.After 200 cycles,the specific capacity can be still stabilized at 885.5 m Ah g^-1.（2）Based on the previous research about Mn₃O₄,a core-shell Fe₂O₃ coated Mn₃O₄nanorods supported on graphene was synthesized by a facial and convenient hydrothermal reaction and heat treatment.The introduction of graphene sheets form a cross-linked 3D conductive network.As an anode for lithium-ion batteries,the designed electrode delivers initial discharge and charge capacities of 1939.2 and 1393.5 m Ah g^-1 respectively,with an initial coulombic efficiency of 71.9%.After 400 cycles,a reversible capacity of 759.4 m Ah g^-1 is obtained.In this process,the capacities increase along with cycles,which may be related with the cross-linked structure and the synergistic effect.（3）A yolk-shell Ti³⁺doped TiO₂ coated Mn₃O₄ nanorod was reasonably designed and fabricated by surface modification.The strong interaction between the Ti³⁺doped TiO₂ shell and Mn₃O₄ core can not only accommodate the volume expansion generated from repeated lithiation and delithiation,but also provide convenient transport channels for lithium ions and electrons and enhance the stability of SEI film by synergistic effects and rapid kinetics.The unique structure is beneficial to high reversible capacity,excellent cycle stability and rate performance.After 400 cycles,the as-prepared anode exhibits an outstanding reversible capacity of 1487.9 m Ah g^-1 and a high capacity retention of 81.67%.（4）In order to study the synergistic effect of binary transition metal oxides,a porous Zn_xCo_3-xO₄ hollow nanobox was synthesized by a simple and quick process using zeolitic imidazolate frameworks as precursors.The porous structure and internal voids can not only provide shorter channels and sites for the transportation and storage of lithium ions,but also buffer the strss and strain caused by volume changes.Additionally,the synergistic effect of the binary metal oxide can improve the conductivity and accelerate the reaction kinetics.The results have also proved by density functional theory（DFT）calculations and the analysis of lithium-storage mechanism.（5）To satisfy the demands about flexible devices,a silicon and carbon coatedα-Fe₂O₃nanorod arrays was grown on the conductive carbon cloth by simple hydrothermal reaction and physical vapor deposition method.The as-prepared three-dimensional flexible electrode can be directily used as an anode for lithium-ion batteries without adding any binders or conductive agent.The nanorod arrays provide larger specific surface area,while the carbon coating effectively suppreses the volume expansion ofα-Fe₂O₃ and silicon during lithiation and delithiation processes.The flexible anode shows superior lithium storage performance,which delivers an initial coulombic efficiency of 76%at a current of 100 m A g^-1.After 500 cycles,the capacity can still be retained at 1060 m Ah g^-1 at 500 m A g^-1,which is 2～3 times that of commercial graphite anode.（6）In order to study the effect between the mass loading of active materials and their electrochemical performances,Sn S₂/graphene/carbon cloth was successfully prepared by a two-step hydrothermal method.The content of Sn S₂ was controlled by altering the fabrication process.The as-prepared flexible film can be directly used as anodes for lithium-ion batteries.The Sn S₂ nanoplates are beneficial to alleviate the stress caused by lithium insertion and provide convenient channels for the transport of lithium ions.Compared with graphene/Sn S2/carbon cloth electrode,the Sn S₂/graphene/carbon cloth with higher Sn S₂ content shows better electrochemical performance.（7）A Ni-Co doped Mo S₂ hollow nanocubes were prepared by a facial process.The hollow nanocubes are composed of Mo S₂ nanoplates,which accelerates the rapid transfer of lithium ions and increases the specific surface area between the electrode and electrolyte.The unique structure can effectively release the stress during charge and discharge process,leading to enhanced cyclic stability.The doped Ni-Co can not only provide more defects and active sites,but also greatly improve the conductivity and diffusion of lithium ions.The as-prepared anode exhibits excellent cyclic stability and rate capability.In summary,the influence of the morphology,structure design,synergistic effect and mass loading of active materials of transition metal compounds to the lithium-storage mechanisms and electrochemical performance were studied in this paper.Besides,flexible anode materials were also explored for lithium-ion batteries,which provides important evidence for energy storage field.